The present invention relates to an electric power steering apparatus for providing a steering assist power with a motor.
In an electric power steering apparatus comprising a motor for generating a steering assist power, the steering assist power is varied according to driving conditions such as steering torque detected by a torque sensor and vehicle velocity detected by velocity sensor. Further, the output of the motor providing the steering assist power is corrected according to angular velocity or angular acceleration of rotation of the motor or steering wheel, steering torque, rate of change of steering torque, and acceleration of change of steering angle of vehicle wheels, whereby the responsiveness to the steering operation is improved to compensate the effect of motor inertia, and the steering system is provided with viscosity to decrease the effect of disturbance. Further, in order to prevent the stability of the control system from decreasing due to the noise in the output signal of the torque sensor and the lag in the system, it has been proposed to vary the phase compensation characteristic for the output signal of the torque sensor according to the vehicle velocity (refer to Patent Documents 1 and 2).    Patent Document 1: Japanese Patent Application Laid-open No. 2003-226252    Patent Document 2: Japanese Patent Application Laid-open No. 2004-98754
However, there is a problem in the electric power steering apparatuses employing the conventional technology that the stability of the control system cannot be sufficiently improved, so that oscillations occur and the effect of disturbance cannot be sufficiently decreased. It is an object of the present invention to provide an electric power steering apparatus that can resolve such a problem.